NWS Forecaster Discussion

Area forecast discussion
National Weather Service New Orleans la
405 PM CST Tuesday Mar 3 2015
Discussion...
another warm/humid day on tap for tomorrow before a significant
pattern shift takes place for the end of the week. Difficult and
flat out frustrating forecast this cycle trying to determine
extent of winter weather threat for local area. If I/ve Learned
one thing over the past few years...it/S that winter weather
forecasting is not fun. It/S even less fun in the south.
Tonight and Wednesday...
expect dense fog to develop again tonight and a dense fog advisory
has been issued for areas generally along and south of the
Interstate 10/12 Corridor. North of these areas...fog is not
expected to be as dense or widespread...but we will monitor the
observations and extend the advisory if necessary. Warm and
humid air mass remains in place over the local area with the cold
front removed to the northwest. Expect scattered showers and
thunderstorms to develop across the region during the afternoon
hours. Set up is not all that favorable for severe weather...but
one or two strong/severe storms can/T be ruled out entirely. Storm Prediction Center
indicating a marginal risk mainly for areas along and north of a
line from Opelousas to Zachary to Tylertown. Any thunderstorms
should come to an end as the instability associated with daytime
heating decreases in the evening.
Wednesday night and Thursday...
overall thinking has not changed much...still looks like a cold
front will move through the area Wednesday night with a
significantly colder but shallow air mass moving into the area
behind it. Latest model guidance suggests front will be a little
bit slower than previous runs indicated which would also delay
arrival of colder air. They/re also slower moving the moisture
out...which means there could still be a window of rain/freezing
rain/sleet mix for far northwestern zones. At this point the main
difference from the previous forecast is the timing of that mix.
With the delay in the arrival of the colder air...it looks like
the main threat timing would begin around 15z and taper off by
about 21z. Regarding accumulation...any freezing rain is expected
to be light and have opted to keep only on hundredth of an inch of
ice accumulation for far northwestern areas during the late
morning Thursday. This forecast comes with the major caveat that
it is a far from clear cut situation. There are some fairly
significant potential hiccups with this forecast scenario. If the
cold air arrives sooner than forecast...the threat timing could
speed up and the area could extend farther south. If the cold air
arrives later...the threat would be delayed or even removed
entirely. Similarly....if moisture lingers longer than currently
forecast...impacts would be greater with potential for additional
icing. If the moisture moves out faster...the threat would be
lessened or removed. Suffice to say anyone in or around the
current expected impact area should closely monitor the forecast
for any changes.
Thursday night/Friday morning...
as if this forecast wasn/T already complicated enough...the GFS
and sref have decided to add another level of complexity. They are
both now indicating that a weak impulse could bring overrunning
precipitation into the area late Thursday night and early Friday morning.
And this happens with temperatures near or below freezing for much
of the area. Mav guidance is bold enough to have a pop of 51
with a low temperature of 30 degrees for kmsy. Problem I see with
that forecast is that skies are forecast to be overcast which
means that majority of cooling has to be from cold air advection.
Agree that there should be some strong cold air advection behind the front...but
with forecast wind speeds over the lake generally 20 kts or
less...there should be at least some warming influence from the
lake. For that reason...am carrying temperatures a few degrees higher
than guidance across the South Shore of Lake Pontchartrain. Of a
bit more concern to ME at this point will be areas along the North
Shore and eastward across the Mississippi coast. In these
areas...feel that freezing temperatures are far more likely...and
am including freezing temperatures in the forecast. If any precipitation were to
still be falling in these areas...it would likely be as freezing
rain. With uncertainty running high and no support from the
Euro...NAM...or CMC...have opted to keep these areas dry for the
time being. Again...anyone in these areas should pay particular
attention to the forecast and how it evolves over the next couple
days as this is a low probability...but potentially higher impact
situation.
Saturday through Monday...
lots of disparity in the different model forecast going into next
week concerning the next system. GFS continues to be the faster
of the two major models with the Euro being a bit slower. Overall
forecast is generally a blend of the two with a slight bias toward
the GFS.
&&
Aviation...
widespread fog is reducing visibilities to one quarter mile or less
at the terminals is expected again tonight into tomorrow morning.
Currently MVFR and some VFR conditions are over the area and expect
ceilings to fall again tonight as fog and status develops. Expect IFR to
periods of vlifr overnight at terminals and conditions will improve
as the the sun rises tomorrow morning.
&&
Marine...
another night of dense fog is expected tonight and a dense fog
advisory has been issued. This fog should mix out during the middle
morning. Moderate to strong onshore flow will develop during the
daytime in response to a tightening pressure gradient ahead of a
cold front. The front will sweep through the waters early
Thursday...resulting in a period of strong offshore winds of 25 to
30 knots and seas of 6 to 9 feet in the open Gulf waters. These
Small Craft Advisory conditions will persist through Thursday
night. As the surface high settles over the deep south on
Friday...gradient flow will ease back into exercise caution range
of 15 to 20 knots. A general northeast wind of around 15 knots
will remain in place through the weekend as a surface high remains
parked over the deep south and the pressure gradient over the Gulf
remains fairly constant. Seas will range from 3 to 6 feet in the
open Gulf waters during this period.
&&
Decision support...
dss code...blue.
Deployed...none.
Activation...none.
Activities...slurry support
dense fog advisory
Decision support services (dss) code legend
green = no weather impacts that require action
blue = long fused watch/warning/advisory in effect or high
visibility event
yellow = heightened impacts with short fused
watch/warning/advisory issuances; radar support
Orange = high impacts - slight to moderate risk severe; nearby
tropical events; hazmat or other large episodes
red = full engagement for moderate to high risk severe
and/or direct tropical threats; events of National
significance.
&&
Preliminary point temps/pops...
mcb 76 64 78 35 / 40 20 40 80
btr 78 66 80 38 / 40 20 30 70
asd 74 61 77 45 / 20 20 30 50
msy 77 64 79 47 / 30 20 30 50
gpt 71 61 74 48 / 20 20 40 50
pql 72 60 75 50 / 20 20 40 50
&&
Lix watches/warnings/advisories...
la...none.
GM...none.
MS...none.
GM...none.
&&
$$
Aviation...13/mh
rest of discussion...95/dm